In radiography the interior of objects is reproduced by means of penetrating radiation, which is high energy radiation belonging to the class of X-rays, y-rays and high-energy elementary particle radiation, e.g. .beta.-rays, electron beam or neutron radiation. For the conversion of penetrating radiation into visible light and/or ultraviolet radiation luminescent substances, called phosphors, are used.
In a conventional radiographic system an X-ray radiograph is obtained by x-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
For use in common medical radiography the X-ray film comprises a transparent film support double-side coated with a silver halide emulsion layer. During the X-ray irradiation said film is arranged in a cassette between two X-ray conversion screens each of them making contact with its corresponding silver halide emulsion layer.
To improve image definition single-side coated silver halide emulsion films are exposed in contact with only one screen and are applied in e.g. mammography, autoradiography and in particular fields of non-destructive testing (NDT) known as industrial radiography. An autoradiograph is a photographic record formed through the intermediary of penetrating radiation emitted by radioactive material contained in an object, e.g. microtome cut for biochemical research.
Phosphors suited for use in the conventional radiographic system must have a high prompt emission of fluorescent light on X-ray irradiation and low afterglow in favor of image sharpness. The relationship between resolution and speed of X-ray intensifying screens is described e.g. in Med. Phys. 5(3), 205 (1978).
Well-known phosphors for use in intensifying screens are tantalate phosphors of the monoclinic M' structure as claimed in EP-Specification 011 909 B1 and more recently in U.S. Pat. No. 5,064,729 from Du Pont, wherein an improved phosphor efficiency and purity of said tantalate phosphors is achieved by reducing the level of contaminants in the phosphor reaction mixture. The preparation of rare earth tantalate and niobate phosphors showing low afterglow by mixing a diacid metal and an alkali to the basic materials has been disclosed by Nichia Kagaku in EP-Specification 202 875 and U.S. Pat. No. 4,959,174.
Other interesting phosphors for use in intensifying screens are rare earth activated alkaline earth fluorohalide phosphors as has been patented by Philips in U.S. Pat. No. 4,157,981 and 4,075,495.
Recently the use and preparation of a phosphor mixture of a divalent barium fluorobromide and a niobium doped yttriumtantalate for X-ray intensifying screens has been disclosed in U.S. Pat. No. 5,077,145. Advantages mentioned are the production of less cross-over light, a better graininess and sharpness and a reduced exposure amount.
Although the phosphors could be mixed in a variety of ratios to obtain screens for the known speed classes as defined in ISO/DP9236 it has been experimented that in using such mixture of phosphors image quality reduces (especially sharpness) as sensitivity increases.
A problem encountered on manufacturing screens from a mixture of the above phosphors for the different speed classes is that a favourable relation between speed and sharpness obtained for screens belonging to one speed class cannot be realized for screens of another speed class. Especially the transition from a lower to a higher speed class leads to a loss in maximum obtainable properties, in particular high speed/sharpness relationship, and a declination of the so-called "technological axis", reflecting said relationship.